Ammonium sulfate nitrate (ASN), one of the first synthetic fertilizers, has been in continuous use for nearly 100 years providing the important primary and secondary nutrients, nitrogen and sulfur. Nitrogen is provided in part through the nitrate ion, desirable because it is readily absorbed by many plants and promotes early growth.
Exemplary ASN fertilizers include double salts of ammonium nitrate and ammonium sulfate having the formula NH4SO4.2(NH4NO3) (known as a 2:1 double salt) and NH4SO4.3(NH4NO3) (known as a 3:1 double salt), such as disclosed in U.S. Pat. No. 6,689,181, which is incorporated herein by reference in its entirety. The term “double salt” as used herein means a chemical compound composed of ions from two precursor compounds whose crystal structure is distinct from those of the precursor compounds. The molar ratio of precursor compounds in a double salt is in the proportion of small integers, e.g., 1:2, and is not continuously variable as in a solid solution. Thus, the term “double salt of ammonium nitrate” means a combination of ammonium nitrate and another compound, such as ammonium sulfate, in such a way as to form new compound which may be crystallographically distinct from either of the constituents. Additional double salt compositions are disclose in U.S. Pat. Nos. 8,075,660, 8,721,760, and 8,814,977, the disclosures of which are incorporated herein by reference in their entirety.
A double salt of ammonium nitrate and ammonium sulfate consists of small ammonium sulfate crystals imbedded in a matrix of the other constituents. A double salt is to be distinguished from a mixture of free particles. The bulk of the ammonium sulfate crystals are approximately the same size as the initial ammonium sulfate particles, but upon solidification about 5 wt. % precipitate as crystals of less than about 2 micrometer dimension. The crystals of ammonium sulfate are dispersed in the matrix in a uniform manner. The small size and uniform dispersion of the ammonium sulfate crystals in the 1:2 double salt significantly enhance the stability of the product against detonation hazard. The '181 patent recognized the benefit of the 2:1 double salt as being a more stable compound. The 2:1 double salts are useful as fertilizers, have reduced moisture sensitivity, are not considered hazardous materials under Title 49 of the Code of Federal Regulations, “Transportation”, Part 172, “Hazardous Materials Table”, Oct. 1, 2000, and are not classified as oxidizers under United Nations Recommendations on the Transport of Dangerous Goods, Manual of Tests and Criteria, 1995”, “Section 34, Classification Procedures, Test Methods and Criteria Relating to Oxidizing Substances of Division 5.1”. Additional examples of methods of producing 2:1 double salts are provided in U.S. Pat. No. 8,623,315, which is also incorporated by reference herein in its entirety.
ASN fertilizers are generally produced by granulation of ammonium sulfate (AS) solids with ammonium nitrate (AN) liquors. Exemplary granulation methods are disclosed in U.S. Patent Application Publication No. 2013/0192323, which is incorporated herein by reference in its entirety. Alternatively, ASN fertilizers may be processed as a melt slurry from either the constituent fertilizers (ammonium nitrate and ammonium sulfate) or, alternatively, from the reaction products of sulfuric/nitric acids with anhydrous ammonia. There are several alternatives for converting the melt slurry into solid fertilizer particles with the physical properties desired by various markets.
Prilling is a process in which a liquid melt is gently streamed from the top of a tall structure with counter-current cooling air. Surface tension divides the stream into individual droplets which solidify before reaching the tower bottom. An illustrative example of prilling may be found in U.S. Pat. No. 7,175,684, which is incorporated by reference herein in its entirety.
Pastillation is similar to prilling in that the melt is converted to a droplet and then solidified. However, it differs from prilling in two distinct aspects. First, rather than relying on surface tension to size the droplets, the droplets are portioned mechanically thus achieving very high uniformity in size. Second, rather than dropping the droplet through cooling air, the droplet is applied to a water-cooled metal belt. The heat is removed through the belt and solidified particles fall off the belt at its end. An illustrative example of pastillation may be found in U.S. Pat. No. 7,985,393, which is incorporated by reference herein in its entirety.
In one type of granulation process, a melt slurry is sprayed onto a moving bed of granules. The melt slurry both coats and agglomerates bed granules to increase in size. In some embodiments, the granules are discharged to a dryer drum which, if included, provides additional rolling time for the granules. The granules pass to a screening operation where the product cut is recovered and the under-sized and oversized material is recycled back to the granulation drum.